LeetCode - Add Binary | Darren's Blog

Given two binary strings, return their sum (also a binary string). For example, given a = "11" and b = "1", return "100".

public String addBinary(String a, String b) {

        int m = a.length(), n = b.length();

        StringBuilder result = new StringBuilder();

        int carry = 0;

        int i = m-1, j = n-1;

        // Sum the two digits at the same position and the carry, and get the

        // resulting digit in that position, and update the carry

        for (; i >= 0 && j >= 0; i--, j--) {

            switch (a.charAt(i)-'0'+b.charAt(j)-'0'+carry) {

                case 0: result.insert(0, 0); carry = 0; break;

                case 1: result.insert(0, 1); carry = 0; break;

                case 2: result.insert(0, 0); carry = 1; break;

                case 3: result.insert(0, 1); carry = 1; break;

            }

        }

        // b has no digit in such significant position

        for (; i >= 0; i--) {

            switch (a.charAt(i)-'0'+carry) {

                case 0: result.insert(0, 0); carry = 0; break;

                case 1: result.insert(0, 1); carry = 0; break;

                case 2: result.insert(0, 0); carry = 1; break;

            }

        }

        // a has no digit in such sifnificant position

        for (; j >= 0; j--) {

            switch (b.charAt(j)-'0'+carry) {

                case 0: result.insert(0, 0); carry = 0; break;

                case 1: result.insert(0, 1); carry = 0; break;

                case 2: result.insert(0, 0); carry = 1; break;

            }

        }

        // Another "1" in the most significant position in case carry=1 after the above steps

        if (carry == 1)

            result.insert(0, 1);

 

        return result.toString();

    }

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